Cancer patients are sometimes quite sick and usually need to lie on their backs for radiation treatment. Also, the patient's anatomy shifts markedly from supine to prone positions. In order to irradiate the target volume from different directions without turning the patient over, 360.degree. rotation of the gantry is needed. For convenience in setting up the patient, the isocenter around which the equipment rotates should not be too high above the floor. Adequate space must be provided between the isocenter and the radiation head for radiation technologist access to the patient and for rotation clearance around the patient. This leaves a quite limited amount of space for the various components and the radiation shielding in the radiation head, and particularly for the magnet system. To a significant extent, the design challenge over the years has been to stay within this space while making major advances in clinical utility of machines. (See Ginzton et al, "History of Microwave Electron Linear Accelerators for Radiotherapy", Int.-J. Radiation Oncology Biol. Phys., Vol. 11, pp 205-216, 1985; Karzmark, "Advances in Linear Accelerator Design for Radiotherapy", Med. Phys., Vol. 11 (2), pp. 105-128 (1984).)
Megavoltage radiotherapy traditionally employs divergent X-ray beams. For example, a 10 cm.times.10 cm field at the tumor position at isocenter 100 cm from the X-ray source focal point would correspond to a 9 cm.times.9 cm field at the patient's skin 10 cm above the tumor center and 90 cm from the X-ray source. This divergent beam technique causes a number of difficulties. For example:
(1) For 6 MeV X-rays and the above example of 9 cm.times.9 cm field at 90 cm source-skin distance in 100 cm source-axis distance (SAD) isocentric treatment (see curve A of FIG. 16), the dose at 10 cm depth is 65.4% of the maximum dose (100%), which occurs 1.5 cm below the skin. (In isocentric treatment the tumor is at SAD.) If the 6 MeV X-rays were parallel instead of divergent, this dose at 10 cm depth would be 78.1% of the maximum dose, which is equivalent to 17 MeV divergent X-rays having 100 cm SAD. Similarly, the depth-dose of 10 MeV parallel X-rays (infinite source-skin distance (SSD), see curve D of FIG. 16) at 10 cm depth is equivalent to the depth-dose of 24 MeV divergent X-rays having 100 cm SAD. (Depth-dose is the dose at a depth, expressed as a percentage of the maximum dose, both on the axis of the radiation beam.) Thus, much of the penetrative quality of a conventional X-ray beam is lost because of its divergence. To regain this penetrative quality using conventional means requires building a much higher energy and hence, more complex and costly accelerator.
(2) The divergent rays create difficult treatment planning problems and create the potential for patient over-dose or under-dose in regions where fields abut. This problem is compounded when the abutting fields are at different gantry angles (e.g., opposing lateral fields to treat the breast and abutting anterior fields to treat lymph nodes outside the primary breast field in the axillary, supraclavicular and mediastinal regions.)
(3) X-ray computerized tomography scans are in parallel slices and these are now used for treatment planning in the central plane of the field. Converting this parallel plane image data into beam's eye divergent view data (to simulate conventional divergent X-ray treatment beams) for three dimensional treatment planning is a complex and time consuming computational task involving expensive digital equipment.
(4) The usual treatment field shapes result in a three-dimensional treatment volume which includes considerable volume of normal tissue, thereby limiting the dose that can be given to the tumor volume. The irradiation dose that can be delivered to a portion of an organ of normal tissue without serious damage can be increased if the size of that portion of the organ receiving such radiation dose can be reduced. Avoidance of serious damage to the organs surrounding and overlying the tumor determines the maximum dose that can be delivered to the tumor. Cure rates for many tumors are a steep function of the dose delivered to the tumor. Techniques are under development to make the treatment volume conform more closely to the shape of the tumor volume, thereby minimizing the product of volume and dose to normal tissue, with its attendant effects on the health of the patient. This can permit higher dose to tumors or can result in less damage to normal tissue. These techniques involve moving the X-ray jaws during treatment or using multi-leaf jaws. Variable blocking of internal portions of the field over the range of gantry angles is quite difficult in such conformation therapy. And the exposure times are long and radiation shielding of present machines is inadequate. But the main deterrent is the excessive time required for three-dimensional treatment planning; and this restriction will be relieved in preparing parallel beam treatment plans from parallel beam CT data.
(5) In conventional radiotherapy machines the distance from the X-ray source to the gantry rotation axis is typically 100 cm in order to provide room for the field flattener, full field dual ionization chamber, light field mirror, X-ray jaws, X-ray field compensator, wedge filter and shadow blocks, and still leave adequate clearance between the patient and the holder for these accessories. At 10 MeV, the X-ray lobe is quite narrow, requiring large attenuation on axis relative to the edges and corners of the field. The X-ray transmission of the field flattener is typically 24% at 10 MeV. X-ray intensity decreases as the square of distance from a point source. The long source-axis-distance and the poor field flattener transmission waste X-ray intensity.
(6) The advantage of small penumbra of X-ray fields is well known for linear accelerators, in permitting protection of nearby radiation sensitive organs. (The penumbra is the region at the periphery of the radiation field where the dose falls rapidly; typically the distance from 80% to 20% of the dose on the axis of the radiation field, measured in a plane at a given depth.) However, this small penumbra is obtained in the treatment plan only in single port fields and in the plane at the isocenter with opposing port fields. In planes displaced along the beam axis from isocenter in opposing port therapy, the X-ray divergence causes the isodose lines at the edges of the field to spread apart. For example, with opposing 20 cm.times.20 cm fields at 100 cm SAD, this divergence increases the typical 6 mm accelerator beam penumbra to about 12 mm (20% to 80%) in the planes displaced .+-.5 cm from isocenter. This increases the difficulty of missing critical organs at the edge of the field in planes above and below the tumor mid-plane while still providing full dose throughout the cross-sectional area of the tumor in these planes.